1. Field of the Invention
The present invention relates to a binary direct sequence (DS) spread spectrum (SS) system, particularly a DS CDMA system, working in an interference-free mode by employing binary spreading sequence sets having zero correlation zone (ZCZ) properties, so as to eliminate the multiple access interference and other counter measures encountered in the system.
2. Description of the Background Art
Future digital communication systems will be characterized by high throughput, integrated services, and flexibility. The direct sequence (DS) spread spectrum (SS) systems, especially DS-SS code-division multiple access (CDMA) approaches have been proposed for a variety of digital cellular mobile and wireless personal communication systems. Cellular CDMA systems offer a range of potential advantages over TDMA or FDMA systems, such as high spectrum efficiency, graceful degradation, multipath resistance, inherent frequency diversity and interference rejection, and so on.
A typical direct sequence (DS) spread spectrum system, as shown in FIG. 1, consists of modulator (1) and demodulator (2), where the spreading consists of multiplying the input encoded and interleaved data by a binary spreading sequence (3), and the despreading consists of multiplying the received signal with the same binary spreading sequence (3), followed by a low pass filter (LPF), decoder and de-interleaver. The bit rate of spreading sequence for each user is normally much higher than the input encoded data bit rate, and therefore spreading the system spectrum. The ratio of spreading sequence bit rate to input data rate is called spreading factor (SF). When the signal is received at the receiver, the spreading is removed from the desired signal due to the impulsive-like autocorrelation property of the spreading code. When despreading is applied to the interference generated by other user's signals, there is no despreading. That is, each spread spectrum signal should behave as if it were uncorrelated with every other spread signal using the same band. Therefore, CDMA codes are designed to have very low autocorrelation sidelobes (preferably zero sidelobes) and very low cross-correlations (preferably zero cross-correlations).
Given a sequence set {a(f)n} with family size M (the number of sequences in the set), r=1,2,3, . . . ,M, n=0,1,2,3, . . . ,L−1, each sequence is of length L, then one can define the following periodic Auto-Correlation Function (ACF, r=s) and Cross-Correlation Function (CCF, r≠s)             R              r        ,        s              ⁡          (      τ      )        =            ∑              n        =        0                    L        -        1              ⁢                  a        n                  (          r          )                    ⁢              a                  n          +          τ                          (          s          )                    where the subscript addition n+τ is performed modulo L.
In ideal case, it is required that the ACF should behave as an impulse, and the CCF should be zeros everywhere. However, it has been proved that it is impossible to design a set of spreading codes with ideal impulsive autocorrelation functions and ideal zero crosscorrelation functions, thus resulting co-channel interference in practical CDMA systems. Generally speaking, the code length L, code family size M, the maximum autocorrelation sidelobe Ramax, and the maximum crosscorrelation value Rcmax are bounded by certain theoretical limits, such as Welch bound, Sidelnikov bound, Sarwate bound, Levenshtein bound, etc.
Apart from the local noise that is irreducible, there are three kinds of interference which affects the DS CDMA system capacity and system performance, that is, intersymbol interference (ISI), multiple access interference (MAI) and adjacent cell (or channel) interference (ACI). However, all the three kinds of interference can be reduced or even eliminated by employing good multiple access spreading codes.
In order to reduce the interference encountered in the DS CDMA system, a number of patents have been published. However, most of the approaches published so far use sophisticated interference cancellers, fast and accurate transmitting power control, variable-rate transmission, and other complicated techniques at the receiver. One previous patent, PCT/CN98/00151 (CN1175828A), uses ternary spreading sequences with limited number of sequences and zero correlation region; another patent, PCT/JP97/03272(JP271858/96), adopts spreading sequences with comb-shaped spectrum.
The present invention was devised in light of the above facts and the object of the present invention is to provide a binary coded direct sequence (DS) spread spectrum (SS) communication system which is characterized by the fact that the binary spreading sequence set of length L and size M used in the system having a zero correlation zone ZCZ in their periodic autocorrelation functions on both sides of the zero shift and a zero correlation zone ZCZ in the periodic cross-correlation functions including the zero shift between any pair of spreading sequences belonging to the code sequence set; and the said spreading code sequence set has length L=2×2i×10j×26k, where i, j, k can be zeroes or any positive integers, and family size 2<M<L, and zero correlation zone ZCZ≦L/M+1 for any permitted values of i, j, k, or ZCZ=L/M+1 for j=k=0 and any permitted value of i; and includes ZCZ sequences and further includes a means for generating a central frame by multiplying a binary input and said ZCZ sequence(s), and additional guard sequence enclosing the central frame, the additional guard sequences being extracted from parts of the central frame. In this way, the multiple access interference, multipath interferences and other central measures can be eliminated efficiently.
Furthermore, the objects of the present invention are to provide sets of binary sequences with specified length, family size and required ZCZ, so that the system can work in an interference-free mode and can be made adaptive to the channel conditions.
Furthermore, the object of the present invention is to provide a DS CDMA system where the conventional intracell short channelization codes such as Walsh orthogonal codes or variable-length orthogonal codes are replaced with the binary sequences with big zero correlation zone, which makes it possible to eliminate the inter-path interference.
Furthermore, the object of the present invention is to provide a DS CDMA system with intercell synchronous operation where the conventional long scrambling code (also called pseudo-noise code) such as m-sequence is replaced with a binary code with big zero correlation zone, which makes it possible to eliminate the multiple access interference.
Furthermore, the object of the of the present invention is to provide a DS CDMA system with intercell asynchronous operation where the conventional long scrambling codes such as Gold codes or Kasami codes are replaced with binary ZCZ codes as presented in this invention, which makes it possible to eliminate the multiple access interference.
Furthermore, the object of the present invention is to provide an adaptive DS CDMA system by allocating each transmitter a subset of sequences with different ZCZs, so that the transmitter can select a suitable spreading sequence according to channel conditions or system instructions.
Furthermore, the DS CDMA system described above is supplied for down-link communication under the condition of synchronous transmission and for up-link communication under the condition of asynchronous transmission.